1. Field of the Invention
The present invention relates to an optical disc drive for use to read and/or write information from/onto an optical disc by focusing a light beam (e.g., a laser beam emitted from a semiconductor laser diode) on a target track of the disc.
2. Description of the Related Art
Recently, various types of optical discs such as DVDs (digital versatile discs) have been developed as storage media on which a huge amount of information can be stored at a high density. Examples of recordable or writable optical discs include DVD-RAM, DVD-RW, DVD-R, +RW and +R. A read-only optical disc such as a DVD-ROM is also known.
An optical disc includes a number of tracks that are arranged spirally thereon. Each of those tracks is defined as a “land” or a “groove” of a recording film, which has an uneven surface and which is often made of a phase-change-type material, for example. Information is recorded on the recording film, and the information-carrying recording film will be referred to herein as an “information storage layer”. The information storage layer is covered with a protective transparent layer.
An optical disc drive may operate in the following manner in writing information on a recordable (or writable) optical disc or in reading information from a read-only optical disc. First, in writing information on a recordable optical disc, the optical disc drive focuses a light beam (e.g., a laser beam that has been emitted from a semiconductor laser diode, for example) onto the information storage layer of a rotating optical disc through the protective coating. In this case, the optical disc drive changes the intensity of the light beam according to the contents of the information to be written while performing a focus control (i.e., such that the light beam is focused right on the information storage layer) and a tracking control (i.e., such that the beam spot can follow the target tracks on the disc) at the same time. As a result, the light beam is reflected from the recording film at various reflectances, thus recording the variation in reflectance as information. On the other hand, in reading information from a read-only optical disc, the optical disc drive receives a light beam, which has been reflected from the optical disc, at a photodetector while performing a focus control (i.e., such that the light beam is focused right on the information storage layer) and a tracking control (i.e., such that the beam spot can follow the target tracks on the disc) at the same time. In accordance with the output of the photodetector, the optical disc drive reads the information.
However, the light beam may be out of focus with the information storage layer of the optical disc, or off the target track on the optical disc, for some reasons. That is to say, the ability of the optical disc drive to gain access to a target location on the optical disc (which will be referred to herein as the “access performance” of the optical disc drive) may deteriorate in numerous situations. Thus, conventional optical disc drives try to minimize such deterioration in access performance by various techniques. Note that “to gain access to a target location on an optical disc” means to read out desired information from, or to write arbitrary information on, the target location on the information storage layer as used herein.
For example, Japanese Laid-Open Publication No. 2002-140825 discloses a technique of allowing focus jumps only in focus-jumpable areas of an optical disc by collecting management data about the past focus jumps. That is to say, if the optical disc has an area in which an attempted focus jump failed due to a vertical deviation of the optical disc, then that area is classified as a non-focus-jumpable area. On the other hand, Japanese Laid-Open Publications No. 2002-157750 and No. 2003-22545 disclose a technique of starting to correct a spherical aberration when or before a focus jump from a first recording layer to a second recording layer is carried out.
In recent years, demands on optical discs with an even higher density or with a further increased capacity have been on the rise. Optical discs with a transparent layer thickness of 0.6 mm have been popularized. To read such optical discs, optical disc drives including a light source that emits a light beam with a wavelength (which will be referred to herein as a “light source wavelength”) of 650 nm and an objective lens with a numerical aperture (NA) of 0.6 are now on the market. However, to further increase the density and capacity of optical discs, the transparent layer thickness of the optical disc needs to be further decreased from 0.6 mm to 0.1 mm, for example. On the other hand, when the transparent layer thickness of optical discs is decreased, the objective lens of the optical disc drive needs to have an NA that is greater than 0.6 and the light source thereof needs to emit a light beam with a wavelength that is shorter than 650 nm. Thus, research and development is now carried on to achieve an NA of 0.85 and a light source wavelength of 405 nm.
However, if the objective lens of the optical disc drive has its NA increased and the light source thereof has its wavelength decreased to catch up with the increase in the density and capacity of optical discs, then the optical disc drive may have significantly deteriorated access performance. For example, if the access performance of the optical disc drive is significantly affected by a spherical aberration to be caused by the variation in the transparent layer thickness of optical discs or a vertical deviation (e.g., a flutter) occurring while the optical disc drive is accessing a target location on an optical disc, then the optical disc drive cannot perform the focus and tracking controls accurately enough solely by the conventional techniques. In particular, a spherical aberration caused on a beam spot is proportional to the fourth power of the numerical aperture NA. Accordingly, even if the variation in the transparent layer thickness remains the same, an increase in NA from about 0.6 to about 0.85 changes the spherical aberration greatly. Thus, it is impossible to avoid the deterioration in access performance by the conventional techniques.